EP 0145070 and DE4121279 forms the closest pieces of prior art and form the basis of the pre-characterising portion of claim 1.
EP0145070 describes a hammer which comprises a ram (or striker) 26 (using the same reference numbers as EP0145070) which is slideably mounted with the main housing of the hammer and which can he reciprocatingly driven via a pivotal arm 32 which is pivotally mounted at one end about a pivot within the housing. The pivotal arm 32 is pivotally driven by the motor via a pivotal drive mechanism which converts the rotary movement of the motor into an oscillating pivotal movement of the arm 32. The ram 26 strikes a tool shaft 1 which in turn imparts the impacts to the end of a cutting tool.
The problem with the design of hammer mechanism disclosed in EP0145070 is that the amplitude of the oscillations of the pivotal arm 32 cannot be adjusted.
DE4121279 also describes a hammer which comprises a ram 24 (using the same reference numbers as DE4121279) which is slideably mounted within the main housing of the hammer and which can be reciprocatingly driven via a pivotal arm 20 which is pivotally mounted within the housing at one about a pivot 16. The pivotal arm 20 is pivotally driven by the motor via a pivotal drive mechanism which converts the rotary movement generated by the motor into an oscillating pivotal movement of the arm 20. The ram 24 strikes a beat piece 28 which in turn strikes the end of a cutting tool 25.
As with EP0145070, the problem with the design of hammer mechanism disclosed in DE4121279 is that the amplitude of oscillation of the arm 20 cannot be adjusted. Another problem associated with the design is that the method by which the end 21 of the pivotal arm 20 is connected to the ram 24. As can be seen on FIGS. 1 and 3 of DE4121279, the end 21 of the arm 20 surrounds the ram 24. Two ribs 22, 23 are formed on the ram 24 between which the end 21 of the arm 20 can freely slide. Thus the ram 24 can slide within the arm 20, the amount of movement being limited by the ribs 22, 23 ie the arm 20 is non fixedly connected to the ram. This results in a limited range of free movement of the ram 24 relative to the pivoting arm 20. As such the control of the ram 24 during the hammering operation is diminished.
GB2295347 and U.S. Pat. No. 5,337,835 are also relevant pieces of prior art.